Imatinib mesylate (IM, also termed Gleevec) has replaced interferon alpha (IFNa) as the standard of care for patients with newly diagnosed chronic myeloid leukemia (CML) due to higher response frequency, lesser toxicity and better survival. While rapidly killing differentiated CML cells, IM fails to act against more primitive leukemic stem cells and early progenitors. As a result, patients receiving IM are not cured and require life-long treatment that is often compromised by resistance to IM due to mutations in molecular target of IM - the Bcr-Abl tyrosine kinase. There are indications that in order to achieve curative effect in CML patients, a combination of inhibitors of Bcr-Abl and the leukemic stem cell-targeting agents (such as IFNa) might be required. While a few reports on success in patients receiving both types of therapy provide new enthusiasm for the re-introduction of IFNa into the management of CML, the molecular mechanisms that underlie the rationale for combining Bcr-Abl inhibitors with IFNa remain to be delineated. Our preliminary data demonstrates that Bcr-Abl activity stimulates the phosphorylation-mediated downregulation of IFNAR1 - a protein receptor that is central for all of the effects of IFNa on cells. This downregulation might confer a suppression of cellular responses to IFNa reported in CML cells. We propose a hypothesis that signal transduction pathways induced and sustained by Bcr-Abl impede the responses of CML cells to IFNa via inducing phosphorylation-dependent degradation of IFNAR1. We also hypothesize that accelerated degradation of IFNAR1 induced by Bcr-Abl plays an important role in regulating the sensitivity of CML cells to IM and IFNa. In this application, we propose to test this hypothesis by (i) determining the role of Bcr-Abl-induced downregulation of IFNAR1 in the inhibitory effect of Bcr-Abl on cellular responses of cells to IFNa; (ii) delineating the mechanisms by which BCR-Abl down-regulates IFNAR1 and its signaling; and (iii) assessing the role IFNAR1 stabilization plays in the therapeutic efficacy of IFNa and BCR-Abl inhibitors. We anticipate that these studies will provide significant insight into the mechanisms by which CML cells expressing constitutively active Bcr-Abl promote IFNAR1 degradation, suppress IFNa signaling and evade IFN-induced anti-tumor defenses. We also predict that these studies will identify critical regulators of these responses that can potentially serve as novel and important targets for increasing the efficacy of therapy against CML. Finally, the outlined experiments will provide the pre-clinical evaluation for sequence-specific combining of Bcr-Abl inhibitors with IFNa to optimize the existing regimes for CML management.